The present invention relates to safety interlock switches and, more particulary, to a switch for preventing power from being applied to an electrical or electronic appliance (such as a computer) once a service door or panel has been opened.
Safety interlock switches for electrical appliances are well known. Typically, they are found in appliances that have high voltage components, such as residential or commercial appliances that require 220 volt power. The service or access door on such appliances will normally actuate the interlock switch. When the door is closed, the contacts of the interlock switch are likewise closed and power is supplied to the appliance. When the door is opened (such as for repairing the appliance), the contacts of the interlock switch are open and power to the appliance is automatically interrupted to prevent accidental electrical shock.
Computer systems have in the past used power contactors in their power control circuits. Power contactors are essentially latching relays which connect the computer (or other load) to the power source when a start switch is actuated and continue the connection until actuating power is interrupted. Once power is interrupted (whether deliberately by an "off" switch or unintentionally because of power failure), the contacts of the power contactor open and electrically disconnect the computer from the power source until the computer is restarted or "booted up" by an operator or service technician.
More recently, computers have been designed to automatically restart after a power failure, so that there is no "down time" due to the need for a service technician to restart the computer. However, there is a risk with such computers that the power has been interrupted because of maintenance, e.g., repair or replacement of hardware components. If power should be inadvertently resupplied before completing maintenance, the computer can be damaged.
It has been proposed to minimize the risk noted above by providing an interlock switch at the service door of the computer. However, the known interlock switches only provide protection when the service door is open. If the door is opened and then closed, but maintenance not completed, such an interlock switch will not protect the computer against damage if the power should be resupplied.
There has thus arisen the need for an interlock switch or other means which prevents power from being resupplied to a computer when the service door of the computer is open, and also when the service door has been opened and then closed (unless the computer is deliberately restarted).